The present invention relates to circuits for energizing gaseous discharge lamps such as fluorescent lamps or high intensity discharge lamps. More particularly, it relates to a ballast using solid state switches and adapted to energize the lamps with high frequency current. Ballast circuits of this type are normally designed to receive energy from a conventional 60 Hz. cycle as is commonly available, and by means of frequency inversion, generate a higher frequency signal (in the range of 25-100 KHz.) to energize the lamps.
The advantages of high-frequency lamp excitation such as more efficient conversion of electrical energy to light output are well-known. However, in the past, and despite the generally accepted principle that high frequency excitation is more efficient, there have been many attempts at high frequency ballasts, but few have met with commercial success. Even those high frequency ballasts which have been commercially produced have one or more disadvantages to them.
Another important factor in evaluating high frequency ballast circuits is the effect that the excitation current has on lamp life. With the rise in energy costs, both ballast manufacturers and lamp manufacturers have, in the last few years, given increased attention to high frequency excitation. Lamp manufacturers have concluded that lamp life may seriously be diminished if the crest factor of the excitation current is not maintained within certain limits.
When, for example, fluorescent lamps were energized by magnetic ballasts at 60 Hz., the crest factor for lamp current (which is defined as the ratio of peak current to RMS current) was approximately 1.41 because 60 Hz. voltage is sinusoidal.
As lamp manufacturers designed lamps for operation at high frequencies, it became clear that the crest factor of lamp current must be maintained within a desired range. It is believed that the heating effect of lamp current is sufficient to heat the cathode of the lamp (in fluorescent lamps) to the point where it is capable of emitting 1.7 times the RMS current. Circuits which exceed a crest factor of 1.7 necessarily exceed the thermionic emission capability of the cathode, and this results in sputtering of the cathode material and shortening lamp life.
Thus, the requirement for achieving a desirable crest factor in high frequency excitation of fluorescent lamps has become an important criteria if a ballast is to receive commercial acceptance. A desired crest factor can be obtained simply by using large inductors and capacitors to filter the line voltage, but the power requirements of these components make them expensive and somewhat bulky, despite operation at higher frequencies.
A high power factor (ratio of wattage to volt amps supplied) is desirable for the electronic ballast. A low power factor (0.9) is usually caused by high line currents or by currents having large peak to RMS ratios. The larger currents (peak or RMS) must have heavier wire to carry it and will allow fewer fixtures on a single breaker circuit. Utilities are often charging a demand fee based on volt-amps supplied because the reactive power is not measured by watt hour meters yet they must still supply it to the customer.
The preferred embodiment of the present invention, thus, is directed to a high frequency inverter ballast for gaseous discharge lamps which achieves a desired crest factor for lamp current and a high power factor with a relatively simple and inexpensive circuit which does not require large magnetic and capacitive components for filtering the DC supply, yet which has many of the desirable characteristics of other solid state ballast circuits.